Over the years, firearms technology has progressed in many different aspects, but technology of the barrel seems to have lagged behind. Ever since rifling was invented years ago to spin the bullet and increase ballistics performance, the length of various gun barrels has been a very important factor.
Many different situations call for different barrel lengths at different times, and these situations can immediately change the ideal barrel length. Many times a very short barrel of only an inch or two is needed for concealed carry and short range shots.
At other times a standard length pistol barrel is needed for both ease of movement and a reasonable degree of ballistics for medium range shorts. Sometimes a standard length rifle barrel is needed and sometimes a shorter length carbine barrel is the appropriate choice.
For longer distance shots, a very long sniper-type barrel is needed to provide maximum ballistic performance.
A barrel is typically secured to a receiver on a firearm with an annular collar extending downward from the barrel to define a face that can be secured to a receiver with a barrel nut in a screwed connection held by the receiver. A screwed connection is not what is used or described herein.
Barrel extensions are known for converting a larger caliber weapon to a sub-caliber weapon. In order to fire a different caliber in a firearm than what the firearm was initially designed to accommodate, a caliber conversion kit creates a smaller bore. A caliber conversion kit changes the caliber of ammunition a firearm fires, usually from a larger caliber to a smaller caliber so that one can fire less expensive ammunition. Such conversion kits typically use a threaded connection for a supplemental barrel and are not known for an auxiliary barrel of the same caliber as the weapon.